Background. There is no standard definition for goitres extending below the thoracic inlet and no clear guidelines for pre-operative planning of surgery. The aim of this study is a practical classification of retrosternal goitres (RSG) based on the anatomical, radiological shape and size of the thyroid.Methods. Retrospective analysis of all thyroidectomies performed in a referral centre between January 2012 and December 2016. Patients with RSGs had a pre-operative CT scan of neck/thorax. Imaging was reviewed to establish features to predict the difficulty of delivering the goitre through the neck incision and to advise the best surgical approach.Results. 847 thyroidectomies were performed with n=98 involving RSGs. TypeA (n=47) are RSG with a shape of a "cone" or pyramid with the apex pointing down. Cervicotomy is the usual approach. TypeB (n=39) are goitres with a shape of a "pyramid' with the apex pointing up, cervicotomy with +/- manubriotomy or sternotomy +/- thoracotomy maybe required. TypeC (n=6) are thyroid enlargements in the mediastinum connected by a pedicle with the thyroid in the neck. A cervical approach +/- manubriotomy or sternotomy +/- thoracotomy is needed. TypeD (n=6) are true intrathoracic or "forgotten" goitres. Sternotomy is indicated for thyroids in the anterior mediastinum though a thoracic approach for those located in the posterior mediastinum might be needed.Conclusion. The shape and size of goitres is important in carefully planning surgery. CT imaging with cross-sectional reconstruction should be analysed before operation. The proposed classification helps treatment planning and allows comparison of outcomes by anatomical complexity.